1. Field of the Invention
The present invention relates to a disk device, a position control device for positioning a head of the disk device in a target position, and a position control method. More particularly, the present invention relates to a disk device and so on with which the head of the disk device can be positioned in the target position without expending time on a seek operation even when an eccentric disk is used.
2. Description of the Related Art
With a disk device for a disk such as an optical disk or magnetic disk, it is extremely important to position the head in a target position accurately. Eccentricity caused by a discrepancy between the center of servo signals recorded concentrically on the disk and the rotary center of a motor may become a hindrance to such positioning.
When a servo signal is recorded on the disk by an external device using a media servo track writing (STW) method or the like, and the servo signal is read by loading the disk into the disk device, eccentricity occurs if the rotary center of the external device disk and the rotary center of the disk device do not match.
A technique known as virtual circular control, in which control is performed to ignore eccentricity, has been proposed as a technique of avoiding eccentricity (Japanese Patent No. 3,683,155 and Japanese Patent Laid-Open No. 2004-39116, for example).
According to this prior art, virtual circular control is a control method in which the head is moved so as to follow a perfectly circular track relative to the rotary center of the disk.
However, even when virtual circular control is performed, the current position cannot be demodulated correctly when the relative velocity of the eccentric orbit and the virtual circular orbit (the orbit of the head) equals or exceeds a certain value. Hence, the head cannot be positioned accurately in a target position, and a seek operation takes a great deal of time.